Steel reinforced aluminum cable (ACSR) for use as an overhead transmission conductor usually comprises a plurality of aluminum wires helically wound around a steel core, which is also typically formed of a plurality of usually round steel wires stranded together. A plurality of layers of aluminum strands are often used. The electrical strands are of electrical grade ("EC") aluminum, one or more aluminum alloys, or a combination of these, tempered to provide sufficient tensile strength to carry a portion of the suspended cable load.
High-voltage transmission companies face numerous problems in reducing costs and ensuring reliable power transmission to their customers. Among these are enormous losses of power due to electrical line losses, extremely expensive maintenance and replacement costs due to broken wires damaged by vibration and oscillation, and the ability to subject the transmission cables to increased loads beyond those for which the cable system may have been designed, if only temporarily, as occurs during peak load conditions or when used to carry the load of a companion circuit that has been temporarily removed from service for maintenance, etc. The known cable standards and constructions represent a compromise among many competing service requirements, thus selection of cable presents an engineering problem of both considerable difficulty and long-term economic importance. The present invention reduces the complexity of the problem by providing in a single overhead transmission conductor a cable with superior conductivity, lower power losses, and greater ampacity for a given cable cross section, and very desirable service characteristics.
Standard ACSR overhead transmission conductor cable utilizes round electrically conductive wire strands. A portion of the tension resulting from the suspended weight of the cable is normally borne by the conventional ACSR aluminum electrical conductors under normal conditions. Under high temperature or high current-carrying operating conditions which soften aluminum wires, however, the steel strand may carry the entire mechanical tension load; the cable thus stretches and sags. ACSR cable is available in the conventional configuration with round conductor strands, and in reduced diameter to meet a "compact" specification. "Compact ACSR" is commonly found in one of two forms.
In one form, at least one layer of the electrical conductor is die-compacted following the stranding operation to reduce the cable cross-sectional area. U.S. Pat. Nos. 1,943,087 and 3,760,093 teach such processes. In another form, the individual strands used for at least one layer of aluminum conductors are shaped into a more compactly fitting cross section, a plurality of which are then fitted together to form the conductor layer or layers. The preferred cross-sectional shape for one embodiment of the invention is called trapezoidal wire. It is shaped before stranding to form the cable. Each compact cable construction relies on different manufacturing steps, and results in differing finished cable characteristics.
Die-compacted ACSR undergoes shaping forces during the compacting process which result in sharp corners or edges. These are susceptible to arcing or corona formation at higher voltage levels, and thus limit use of the configuration to lower voltage levels.
Trapezoidal wire ACSR is formed by "building up" pre-shaped conductors, resulting in a very dense structure without the mechanical rigidity of die-compacted ACSR. This cable construction can improve the resistance of the wire to aeolian oscillation and galloping, to which such conductors are subjected. Aeolian oscillation is a low amplitude, high frequency vibration that normally occurs due to relatively low wind velocities under 25 kilometers per hour. Galloping, conversely, is a low frequency, large amplitude phenomena. Both galloping and aeolian oscillation can contribute to fatigue and early failure of the conductors in conventional ACSR cable.
As noted, a portion of the tension force is normally carried by the aluminum conductor in ordinary ACSR. However, a condition knownas "tension creep" elongation is known to occur, in which the aluminum conductor portion of the overhead cable stretches over time and permits a degree of conductor sag which may be undesirable. This can increase the load on the steel strand core since the tension force carried by the aluminum conductor is reduced without a reduction in the weight of the aluminum conductor.
Electrically conductive metals used for conductor cables are subjected to complex mechanical and heat treatments in order to arrive at desirable mechanical and electrical characteristics. As is well known, the interaction of the mechanical and heat treatments and the electrical characteristics is extremely complex; this complexity is vastly increased when the metal strands are subjected to the manufacturing process conditions necessary to produce a finished cable, installed for use. Tensioning, bending, and frictional heating of the aluminum conductor strands alter the electrical conductivity and temper thereof, often contrary to the finished effect desired.
U.S. Pat. Nos. 3,813,481 and 3,813,772 ("'481" and "'772") disclose known overhead transmission conductor cable designs in which the aluminum wires are at nearly dead soft temper and the stranded steel core carries substantially all of the tension load. This cable is denominated steel supported aluminum conductor, or SSAC. The '481 patent is believed to represent more recent improvements in overhead transmission conductor cable designs. In the design illustrated in that patent, the aluminum conductor wires are annealed to soft condition such that the stranded steel core carries the tensile load.
The manufacturing process for the SSAC product 100 disclosed in the '481 patent is illustrated in FIG. 4. Conventional 61% IACS aluminum rod 102 is drawn conventionally to wire form in a drawing step 104, then the drawn wire 106 is fully annealed in step 108. This drawn, fully annealed wire 110 is soft and easily subject to damage and must be handled carefully. This careful processing requirement extends to the special stranding step 112, where the conductor wires 110 are overlaid around the steel strand core 114.
Strain and work hardening as ordinarily and inherently occur in the stranding process must be minimized to avoid increasing the temper of the wires unnecessarily, as the finished overhead transmission conductor cable wires are specified as having less than 8500 pounds per square inch (psi) yield strength for 1 percent elongation and must provide at least 61% IACS conductivity in the final product. Therefore, the stranding step 112 described in the '481 patent includes numerous special processing condition requirements which necessitate extraordinary adjustments to the stranding apparatus and significantly slower processing speeds.
These special stranding step 112 requirements include, but are not limited to: applying a lubricant to the surface of the fully annealed aluminum wires, reducing the back-tension on the aluminum wires through the stranding machine, reducing the operating speed of the stranding machine, modifying the wire guides to minimize scuffing (which can cause scratches), enlarging the closure dies which press the annealed stranded wires against the steel core, and reducing the pressure of the closing dies. Even with these special stranding precautions, a degree of hardness is imparted to the aluminum conductor wires which requires careful attention, as the upper limits of the yield strength are prescribed at 8500 psi.
In addition to these uneconomical and difficult requirements and adjustments, extreme care must be exercised to protect the fully annealed wire 106 during the stranding step 108. That is, since the wire is dead soft, the surface is easily scratched or damaged; such scratches are an important cause of arcing and corona in the finished overhead transmission conductor cable. Special care and selection is required for overhead transmission cable intended for higher voltage service.
Of particular interest among the teachings of the '481 patent is that the product is to be subjected to only a single annealing step throughout the cable manufacturing process disclosed. The full anneal is to take place within the time frame illustrated at T11 of FIG. 4; i.e., after the drawing step 104 and before string-up 116 of the finished product is completed by placing it in regular service. Due to the deleterious effects of the high temperatures of the annealing process on the steel strand, the '481 patent teaches that the annealing step 108 is preferably performed within the time frame illustrated at T12 of FIG. 4, that is, after the drawing step 104 and before the special stranding step 112. It will be appreciated by those of ordinary skill in the art that a normal anneal occurring after stranding will negatively affect the performance characteristics of the steel strand.
These special manufacturing requirements add significantly to the cost of manufacturing this SSAC cable. No improvements in conductivity of the completed product are disclosed.